In the production of optical video discs, the information to be recorded on the disc is pressed or embossed into one side of a thin transparent substrate formed of a thermoplastic material such as polymethylmethacrylate, by raising the temperature of the substrate to a softened state at which a stamper bearing the recordings is pressed against the substrate, and the substrate is then cooled in contact with the stamper until it reaches its solid state.
In accordance with the prior art practice, a reflective coating is then applied over the substrate by sputtering or vacuum metallizing techniques in a second apparatus, after the substrate has been removed from the press. After further prior art processing, the resulting video disc with its reflective coating may be assembled with another similar disc carrying other recorded information, into a completed video disc bearing video information on both sides which may be optically reproduced.
The prior art practice of removing the thin transparent substrates from the press, and the temporary storage of the substrates, permits the accumulation of airborne dust and oils on the surfaces of the substrates bearing the recorded video information. Furthermore, the sputtering or vacuum deposition processes used in the prior art for applying the reflective coatings to the substrates are usually imperfect on a microscopic scale, especially on the relatively large surface of a typical video disc (which usually is of the order of 300 millimeters in diameter). Moreover, the prior art processes for applying the reflective coatings are incapable of removing debris already present on the information bearing surfaces of the discs. The result is that there are many microscopic defects in the reflective surfaces of the prior art discs, and these defects in the discs, after all the manufacturing steps have been completed, usually result in visible and audible perturbations in the reproduction of the information recorded on the discs when they are operated on the reproducing apparatus.
A principal objective of the present invention is to eliminate the need to move the embossed discs from the press to a second apparatus in order to apply reflective coatings to the discs, and therefore to eliminate the exposure of the discs to the accumulation of foreign matter during the manufacturing process.
By means of fluid organometallic compounds, the metallic atoms within the molecules of the compounds may be made to form a thin metal coating by deposition on foreign surfaces which are heated to the temperature of decomposition of such compounds, this technique being known as pyrolysis or chemical vapor deposition. Although the technique of deposition of metals by the decomposition of organometallic compounds, or gas plating as it is called, is known, the metals so produced have been used primarily in order to secure high orders of purity. In the practice of the present invention, however, the organometallic deposition process is employed in order to obtain an optical surface of relatively high and uniform reflectivity over a large surface area.
The range of fluid organometallic compounds which may be used in the process of the invention is wide, and although both liquid and gaseous forms may be used, the examples to be described herein are of the type which employ a metallic compound, such as aluminum, in its gaseous form for plating a reflective surface on the substrate of a video disc. The gaseous form of the aluminum may be, for example, alkyl, alkyl halide, alkyl hydride, hydride halide, and the like, compounds; and these will be referred to collectively herein as alkylaluminum, although it is to be understood that other organometallic compounds such as, tetramethyl tellurium, and the like, may be used in the practice of the process of the invention.